Change your domain’s settings to authenticate the sent messages with SPF
(this is generally done after the trial period)

Why? If you wish to have email from your domain sent through a third party service,
you must configure your SPF record to permit delivery from the service provider’s IP addresses.
If you don’t do this, then you risk having email receivers reject all email sent from your domain.

Setting up the RealSender SPF record is easy:
all you have to do is add include:spf.realsender.com to your SPF record, and you’re done.

We automatically sign emails with DKIM, so you don’t need to do anything else.